1. Field of the Invention
The present invention is directed to an apparatus for sensing a physical condition in a living subject of the type having a sensor for acquiring a measurement signal related to blood pressure in the subject.
The invention also relates to a rate-responsive heart stimulator.
2. Description of the Prior Art
For people with various pathological heart conditions or functional defects in different tissues, the heart in particular, measured indications of the patient's state of health constitute desirable information. It is also desirable to have the ability to stimulate the functioning of the heart of patients with heart problems, so the functioning of a healthy heart is simulated as accurately as possible.
As used herein, "physical condition" refers to both the general state of health of the subject as well as the workload (activity level) of the subject, i.e., whether the subject is at rest or under light or heavy physical strain.
Blood pressure is a parameter which reflects a person's state of health and which can also be utilized for controlling a heart stimulator's stimulation of a heart.
In European Application 0 178 528 a heart stimulator is disclosed which can stimulate a heart at a variable rate according to systolic peak pressure or its time derivative. Here, a pressure sensor is placed in the right ventricle to measure blood pressure. Systolic pressure increases when the patient's level of physical activity increases, and an appropriate stimulation rate can be set according to the peak pressure value or the time derivative of pressure.
Systolic pressure is, however, a heart parameter which does not solely depend on the patient's degree of physical exertion, but also is affected by other factors, such as stress, body position etc. In addition, there are natural variations in the systolic pressure of patients. This means that systolic pressure is not a particular appropriate parameter for use in attaining the most natural variation in stimulation rate possible.